In modern and future (cellular) communication systems, inter-band multi-carrier capability of devices such as terminal devices is gaining more attention and importance.
For example, in 3GPP systems, inter-band carrier aggregation represents an inter-band multi-carrier communication framework which shall be supported by devices such as terminal devices. In inter-band carrier aggregation, at least two carriers operating on different (frequency) bands are aggregated together in/for at least one of downlink and uplink.
When multiple carriers operating on different (frequency) bands are combined or aggregated (which is regarded as a carrier combination/aggregation transmission mode herein), such as in inter-band carrier aggregation, intermodulation distortion (IMD) is typically produced due to nonlinearities in active and/or passive components of a device transmitter (in case of uplink transmission), a device receiver (in case of downlink transmission) or a device transceiver (in both cases). Generally, intermodulation distortion components of (m+n)-th order are located at frequencies m*f1±n*f2. For instance, assuming that uplink carriers at frequencies fUL1 and fUL2 are combined or aggregated, a second order intermodulation distortion component is located at one of frequencies 2*fUL1, 2*fUL2, and fUL1±fUL2, and a third order intermodulation component is located at one of frequencies 3*fUL1, 3*fUL2, 2*fUL1±fUL2, fUL1±2*fUL2, and so on.
When at least one (or part) of the intermodulation distortion components falls in a (frequency) band being used for some transmission operation by the device in question, for example a DL carrier, such intermodulation distortion can cause a significant amount of desensitization. Such desensitization is specifically applicable for certain (inter-band) combinations of standardized carriers depending on the frequency relations between uplink and downlink channel definitions.
For certain frequency relations between uplink and downlink channel definitions, (part of) intermodulation distortion components of uplink carrier combinations are produced on top of a downlink carrier and thus destroy the performance thereof, if no power control (for example power restrictions) is in place. Referring to 3GPP uplink and downlink channel definitions according to 3GPP TS 36.104 (Table 5.5-1), for example, aggregating bands (i.e. aggregating component carriers operating on bands) B20 and B8 causes a third order intermodulation distortion component from B20 UL (832-862 MHz) and B8 UL (880-915 MHz) to overlap with B8 DL (925-960 MHz). Furthermore, (part of) intermodulation distortion components can also be produced on top of some non-3GPP Radio Access Technology (RAT), for instance in the 2.4 GHz frequency band (ISM band) used for example by WLAN and Bluetooth. For instance, this is the case for the second order intermodulation distortion component when aggregating bands B4 and B12.
The intermodulation problem as outlined above, i.e. an excessive desensitization of some operating bands/carriers (for example a DL carrier in case of a multi-band UL carrier combination), can be avoided if the intermodulation power of an intermodulation distortion component is sufficiently low as compared with the actual transmission power of that operating band/carrier (for example below a reference sensitivity level). For instance, desensitization could be considered to be not significant when smaller than 0.5 dB.
However, there are currently no means for reliably and efficiently ensuring such a sufficiently low level of desensitization in the context of intermodulation distortion for inter-band multi-carrier combinations or aggregations, particularly whilst avoiding coverage problems and enabling flexible power control for multiple uplink carriers.
Using power restriction values for instructing inter-band multi-carrier power control at terminal devices by a network entity such as a base station is not effective in terms of at least one of coverage and control flexibility. Rather, a controlling network entity using power restriction values for power control instructions will not know exact output powers actually being used for individual uplink carriers at terminal devices based on such instructions.
Thus, there is a desire to improve power control for inter-band multi-carrier capable devices. More specifically, there is a desire to improve power control for inter-band multi-carrier capable devices in terms of at least one of coverage and control flexibility, for example in the context of inter-band carrier aggregation.